The present invention relates generally to jam strobe resolution in a radar system using a monopulse antenna.
The generation of jam strobes, that is the determination of jammers' angular positions, is a typical requirement for surveillance radars. When multiple jammers are present, and this is typical of current specified threats, the issue of jammer resolution obviously arises. Techniques have been developed and applied to deal with this problem for the fixed beam type of mechanically scanning surveillance radars that are in common use today. These techniques generally depend on recognition of the scan modulated jammer power pattern that is generated as the antenna scans past the jammer or jammers. These methods are not suitable for the type of agile beam radars being considered for future surveillance radars that will use electronic step scan and are either totally static mechanically or, if they are rotating, spend only brief dwells in any particular direction. In these cases scan modulation is either non-existent or of too little extent to be of value for resolution purposes. Such radars, in general, will employ monopulse angle measurement capabilities, which actually enhance their ability to generate accurate jam strobes on individual jammers, but which cause problems when it is necessary to resolve closely spaced jammers. There is need for an approach to dealing with these problems and providing a jam strobe resolution capability for monopulse antennas when there are two jammers in the main beam.
It is perhaps worth pointing out that the typical race track stand-off jammer threat frequently produces pairs of closely spaced jammers as they cross one another in the front and back legs of the race track. More than two closely spaced jammers are much less likely to occur.
It is well known (Samuel M. Sherman, "Monopulse Principles and Techniques", Artech House, Chapter 8, Response to Unresolved Targets) that when two fluctuating sources are present within the coverage of a monopulse antenna, the average, or weighted average, of the indicated angle is at the so-called "power centroid" of the two sources. On the other hand, if one source is consistently stronger than the other, the unweighted average indicated angle is that of the stronger source.
Another well-known characteristic of monopulse systems is that the presence of multiple, rather than single, sources within the coverage can be detected (Sabi J. Asseo, Detection of Target Multiplicity using Monopulse Quadrature Angle, IEEE Trans. AES-17, No. 2, March 1981, pp. 271-280) by the measurement of the imaginary part of the difference/sum ratio Im(d/s), as well as the real part, Re(d/s). Re(d/s) is normally used to calculate the source angle, via the appropriate d/s vs. angle calibration relationship. However, if Im(d/s) is large under high signal-to-noise ratio conditions, the presence of multiple sources is indicated and (Re(d/s) is not a good direct measure of their locations.
United States patents of interest include U.S. Pat. No. 4,646,095, to Kanter, which discloses a method of resolving with a monopulse antenna the signals from two sources in the sum beam of the antenna. Those sources may be jammers. This is done in the patent using the ratios of the sum and difference signals. A monopulse radar for resolving jammers is also shown in U.S. Pat. No. 4,042,927 to Helms. U.S. Pat. No 4,107,682 to Boucher et al is concerned with a system including a monopulse antenna along with the combination of sum and difference signals for overcoming the effect of electronic countermeasures. Sum and difference signals along with cross-related or cross term signals are used by Schenkel et al in U.S. Pat. No. 4,219,816 for angular measurement in target tracking radar systems. Jammer suppression using sum and difference signals is taught in U.S. Pat. Nos. 4,459,183 and 4,573,051 to Farina.